1. Field of the Invention
The present invention relates to a semiconductor package, particularly to such a semiconductor package provided with a heat slug, a heat sink or a heat spreader, hereinafter referred to as a "heat slug", made of metal on a bottom surface of a cavity for mounting a semiconductor element.
2. Description of the Related Art
A conventionally known semiconductor package such as PPGA (Plastic Pin Grid Array), PBGA (Plastic Ball Grid Array) or PLGA (Plastic Land Grid Array) is constructed by overlaying a plurality of resin boards, each composed of an electro-insulating substrate made of glass-epoxy resin, glass-polyimid resin, BT resin or others, on which an electro-conductive layer such as a copper foil is layered, with each other, as shown in FIG. 11.
A structure of the conventionally known semiconductor package 10 will be described with reference to FIGS. 11 to 13. An opening 14 is formed in the overlaid respective resin boards 12 so that a cavity 16 is formed for accommodating a semiconductor element. Circuit boards 20a, 20b carrying circuit patterns 18 on both surfaces thereof are formed by etching the copper foil of the respective resin board.
Bonding portions of the circuit pattern 18 are formed in the vicinity of the periphery of the opening 14, for the connection of the circuit pattern to the semiconductor element by a wire bonding method. A size of the opening 14 in the respective circuit board 20a is selected so that when a laminate is formed by overlaying the circuit boards 20a with each other, the bonding portions are exposed outside in every circuit board 20a. Thus, the size of the opening 14 becomes larger as the position of the circuit board 20a, 20b is higher in the cavity 16 of the lamination of the circuit boards.
The circuit board 20a, 20b is adhered to the other circuit board via a adhesive sheet 22 (prepreg). Also, a resist (not shown) is coated on a surface of the circuit board 20a to smooth the surface and enhance the adhesivity between the circuit boards by the adhesive sheet 22. In this regard, if the characteristics of the adhesive sheet allow, the resist may be eliminated.
On the outer surface of the circuit board 20b, there are formed as a part of the circuit pattern 18 a land 26a for connecting an external terminal 24 (a bump in this case), a conductor 26b for connecting a circuit element such as a capacitor or a resistor, or a conductor 26c for mounting a heat slug made of a metallic plate. A protective coating 28 such as a solder resist is provided on the outer surface of the circuit board 20b, while taking a care that the land 26a and the conductors 26b, 26c are solely exposed.
The external terminal 24, the circuit element and the heat slug 30 are mounted to the land 26a and the conductors 26b, 26c, respectively.
Next, a method for mounting the heat slug 30 to the conductor 26c will be described. First, a thermosetting adhesive 32 is coated on the conductor 26c, and the heat slug 30 is placed on the circuit board 20b while taking a care so that a center of the heat slug 30 coincides with a center of the cavity 16 by using a positioning mechanism. Thereafter, the circuit boards 20a, 20b and the heat slug 30 are pressed together under heat, and the thermosetting adhesive is hardened to bond the heat slug 30 onto the conductor 26c of the circuit board 20b.
However, there is a problem in the prior art semiconductor package mentioned above, which is as follows:
A semiconductor element 34 such as a computer chip is mounted to an attachment area A of the heat slug corresponding to the inner bottom surface of the cavity 16 of the semiconductor package.
Generally speaking, the size of the opening 14 provided in the circuit board 20b having the conductor 26c is selected so that a gap B between the semiconductor element 34 and the peripheral edge of the opening 14 of the circuit board 20b is as small as possible to minimize the contour of the semiconductor package 10 as a whole, while taking account of the size of the attachment area A (in other words, the size of the semiconductor element 34), a positioning accuracy of the positioning mechanism, or a displacement for the scrubbing operation when the semiconductor element 34 is mounted to the attachment area A via an adhesive 36 (scrubbing is an operation for removing air bubbles from the adhesive by pressing and moving the semiconductor element on the heat slug 30).
For this purpose, an amount of adhesive 32, a pressing force and a heating temperature used for bonding the heat slug 30 with the circuit board 20b should be strictly controlled. Otherwise, the adhesive 32 positioned between the heat slug 30 and the conductor 26c is liable to squeeze out from the peripheral edge of the opening of the circuit board 20b along the heat slug 30 toward the attachment area A as shown in FIG. 12, whereby an area for mounting the semiconductor element 34 could not be ensured on the heat slug 30. Contrarily, as shown in FIG. 13, when an end of the adhesive 32 closer to the attachment area A does not reach the peripheral edge of the opening 14, there is a gap 38 between the upper surface of the heat slug 30 and the lower surface of the conductor 26c, which might cause trouble in that, when the semiconductor element 34 is sealed in the cavity 16 with a resin after mounted thereto and wire-bonded to the circuit patterns, an air bubble in the gap 34 might form a pit-like air opening on the surface of the sealing resin to disturb the stamping of product number, or if the air bubble remains within the sealing resin, a crack might be generated in that portion later.
However, the adhesive 32 may vary in hardening property in accordance with the difference between lots or storage conditions. Therefore, it is necessary to strictly control the adhesive not only during the bonding operation for factors such as an amount thereof or a pressure and a temperature but also during the storage thereof, which is very troublesome. Even if well controlled, there may be cases, in accordance with the difference between lots, wherein an end of the adhesive closer to the attachment area A exceeds the peripheral edge of the opening 14 of the circuit board 20b (as shown in FIG. 12) or does not reach the same (as shown in FIG. 13). Therefore, it is very difficult to locate the end of the adhesive 32 within a predetermined allowable limit C relative to the peripheral edge of the opening 14 as shown by dotted lines in FIG. 13.